1. Field of the Invention
The present invention is related to a capacitance sensing circuit, and more particularly, to a capacitance sensing circuit with adaptive detecting range for use in a touch panel.
2. Description of the Prior Art
Liquid crystal display (LCD) devices with thin appearance have gradually replace traditional bulky cathode ray tube (CRT) displays and been widely used in various electronic products. With rapid shrinkage in size, there is less room for traditional input devices such as keyboards or mice. Therefore, touch panels providing tactile inputs and display function have become more and more popular. There are various types of touch panels, such as resistive, capacitive, surface acoustic or infrared. Among those, capacitive touch panels detect capacitance variations corresponding to changes in static electricity caused by tactile inputs from a human finger or a stylus, thereby capable of determining the actual location of the touch action.
In a traditional capacitance sensing circuit, an analog capacitance-detecting circuit is normally adopted for detecting a measured capacitance CSENSE of a panel, based on which a corresponding digital signal is calculated using a counter. The measured capacitance CSENSE includes a stray capacitance CPANEL which is inherently present in the panel and a touch capacitance CFINGER which is present due to a touch action. Since panel size normally increases with resolution, the inherent stray capacitance CPANEL also increases accordingly. When a finger is in contact with the panel, the increase in the touch capacitance CFINGER is insignificant compared to the stray capacitance CPANEL of the entire panel (CPANEL>>CFINGER). While the capacitance variations due to other noises are also much larger than that in the touch capacitance CFINGER, the prior art capacitance sensing circuit may not be able to provide accurate capacitance measurement. On the other hand, the capacitance input range is normally set within the optimized linear region of the capacitance sensing circuit. However, the prior art capacitance sensing circuit may operate in the non-linear region if the capacitance input range varies with humidity, temperature, operational environment, process or device aging, which largely reduces image resolution.